1. Field of the Invention
This invention relates to digital transmission systems and particularly to cutover apparatus and methods for switching from one transmission media to a second transmission media.
2. Description of the Related Art
Currently telephone companies provide their customers with a digital point-to-point carrier services that can transmit up to 1.544 million bits per second. The existing digital point-to-point carrier services use copper twisted pair wiring which was designed to carry voice grade communications with frequencies of 300 hz to 3 khz, rather than the high frequencies of digital signals which range between 56 kilobits per second to 45 megabytes per second. With the decreasing cost and the increased reliability of fiber optic communication systems, telephone companies are upgrading customers to fiber optic lines. Using a light signal rather than an electrical signal, fiber optic systems are not subject to signal degradation caused by external electromagnetic fields. New fiber optic systems also incorporate backup systems built to automatically handle line failures. Since customers find these features desirable, telephone companies are upgrading their customers to fiber optic systems in order to stay competitive in the digital services market.
Unfortunately, to make an upgrade from copper twisted pair wiring to fiber optic lines, the telephone company must schedule a service outage with the customer. Many of these customers have come to rely on their digital communications for business. A super-computing center or a bank can lose a significant amount of time and money while their service is down. As a result, obtaining a release for down time is very difficult. If the telephone company does manage to schedule down time, the customer usually requires that the down time occur during off hours, thus, forcing the telephone company to pay its workers overtime to perform the upgrade. Also, fiber lines can serve 28 customers, so the telephone company would have to schedule the upgrade for all 28 customers.
Switching is also necessary to upgrade from other transmission media such as microwave to laser or other media. Similar upgrade scheduling problems occur for these other media.
It is desirable to eliminate these problems while finding a method to upgrade the customer from one media to another without causing any down time. This would allow telephone companies to build the new fiber optic service and then during regular hours switch to the new service without interruption to the customer. Further, it is desirable to make the switch as rapidly as possible so that the customer's receiver will not detect a change from one media to another.
Another requirement is to maintain signal quality. All transmission lines have a unique characteristic impedance. This property is a function of the geometry of the transmission wires used. If a transmission line has a resistive load at each end equal to the characteristic impedance of the line, a signal transversing the transmission line will be completely absorbed when it reaches the end of the line. If the transmission line is not terminated by its characteristic impedance the signal will not be completely absorbed at the end and will reflect back to the source. The reflection will interfere with and degrade the signal being transmitted. Thus, it is necessary to have a method of switching from one transmission media-to another transmission media while maintaining the proper resistive load at each end of the transmission media equal to the characteristic impedance of the transmission media.
Accordingly, there is a need in the art for apparatus and methods for switching from one transmission media to a second transmission media.